Offshore marine structures of the type contemplated are often used for drilling wells and producing hydrocarbons from reservoir sites beneath varying water depths. Depending on the location of these sites, the tidal difference in the water level can vary from several feet, to perhaps in excess of 5 feet.
Platforms and marine structures of this type must be capable of withstanding adverse weather situations. Extreme and stormy conditions are countered where waves can be greater than 75 feet high above mean water level.
Such platforms therefore are fixed to the underwater site by piling or similar means which will give them a positive, firm hold. They must also support the weight of the deck together with its ancillary equipment, as well as resist the foul weather.
In any instance, means must be provided on the structure for accommodating work boats and other floating craft which pull alongside. The latter are a necessary adjunct to the operation of any offshore platform since they carry supplies and tools to assure a continuous operation. Such work boats also carry personnel and crews which are usually rotated on a cyclical basis.
In summary, the use of boats in conjunction with offshore operations is a necessary expedient in achieving functions which cannot be performed by helicopter.
To function properly, any boat landing attached to offshore structures should be adjustable to accommodate the boat regardless of the tides and the general condition of the water about the platform. However, due to the extremely rough conditions which are to be expected, it is found to be highly impractical to make a boat landing which is operable and which can be adjusted to a prevailing condition.
It is expeditious therefore to install the structure's boat landing in such manner that it will accommodate approaching boats under situations which normally might prevail at the platform regardless of the mean water level.
The usual precedure for providing an offshore working site with a marine structure capable of drilling or producing hydrocarbons, is to design and fabricate the structure at a boat yard or similar onshore facility. At this point the steel supporting jacket is assembled in such manner that enough of its members are sealed and buoyancy controlled that the platform can be transported to the site by barge. It is then controllably sunk in a substantially vertical or upright position. Once the structure is resting on the ocean floor, it is fixed there by the usual piling which extends through the structure's legs and into the substrate. The piling is embedded a sufficient distance to firmly anchor the unit.
Normally the height of a marine structure can be accurately determined in accordance with the condition of the substrate into which the piles will be driven. It is virtually impossible, however, to accurately determine the exact mean water level along the platform legs. This is true since the entire unit will tend to sink into the floor of the offshore site an uncertain distance during installation.
Since the ultimate level to which the water will rise on an installed platform's legs is unknown at the time of construction, it is difficult to initially install a boat landing which will be ideally suited on the structure for normal usage.
For example if the landing is installed too low on the platform legs, should the latter sink an inordinate amount into the ocean floor a fixed boat landing might be underwater at all times and of minimal usage. On the other hand, if the landing is initially installed too high, it could be too far above the deck of approaching vessels and again be of minimal usage from a practical point of view.
What is presently provided in an offshore platform to overcome the above enumerated problems is a marine structure, which includes a boat landing that depends from one side thereof. The boat landing or stage is partially installed on the structure at the onshore fabrication yard and prior to being barged to the offshore site.
In anticipation of the installation, at least two of the marine structure's upstanding legs are furnished with a tracking system that will operably receive a boat landing and yet permit subsequent vertical adjustment to a desired elevation. After the structure has been piled into its permanent location, the boat landing can then be moved to a height compatible with the actual mean water level on the platform's legs.
It is therefore an object of the invention to provide an offshore structure of the type contemplated having a boat landing or stage which is properly positioned relative to the level of water about the platform.
A further object is to provide a boat landing for an offshore structure that is installed subsequent to the structure being fixed to the floor of an offshore working site.
A still further object is to provide a boat landing that is installed at an offshore site by vertically adjusting the boat landing substructure or tracking system on a provisional basis prior to the platform being fixed to the ocean floor.